1. Objectives of the Invention
It is an objective of this invention to provide an improved method to cure polar unsaturated polyester, or polyimides. Other objectives will be obvious to those skilled in the art.
2. Prior Art
It is well known that unsaturated polyester resins can be cured to their final form by the copolymerization of the unsaturated polyesters with one or a mixture of vinyl-type monomers (which can also serve as the solvent for the polyesters). The result is a highly usually crosslinked polymeric substance. The crosslinking is achieved with the aid of an initiator which is also termed a "catalyst" or "accelerator". Initiation of curing is done conventionally by including an addition polymerization or copolymerization of the various species of unsaturated carbon-to-carbon double bonds in the resin composition. This chain reaction is triggered by one of several means. Simple application of heat (thermal activation) can be used. Normally, this means of initiation is uncontrollable and, in addition, leads to short shelf life for compositions stored even at room temperature. Because of shelf life problems, the compositions are (in practice) treated with thermal initiation inhibitors, and therefore heat is not used in known commercial operations as the sole means of activating the crosslinking reaction.
A second known means of triggering the vinyl addition polymermization and crosslinking of polyesters is through the use of certain wavelengths of light. This technique is also considered to be impractical for the commercial curing of these types of resins (see Lawrence, John R., Polyester Resins, Reinhold Publishing Corporation, New York, 1960, page 91).
The most widely used means of initiating the cure of unsaturated polyester resins is the use of so-called chemical free radical "catalysts" which decompose into highly active free radicals that initiate the free radical polymerization and crosslinking.
There is generally little or no homopolymerization of the vinyl-type monomer, .e.g, styrene, unless there is a very high proportion of the vinyl-type monomer present.
Unsaturated polyesters have also been cured by gammairradiation of the polyesters whereby there is random intramolecular crosslinking with, in some cases, radiation induced chain growth.